Is it possible to convert a PCI-e x1 slot into an old PCI slot? If so, how?
Well after a little digging around (which I guess i should have done first) the truth of the matter is, yes, there actually are. There are little cards that you can slip into the PCI-e x1 slot to convert it to a PCI slot. Why would anyone want to do this you ask? well, many companies still make products intended for PCI slots, not PCI-e. It was the graphics card companies that lead the rush twords PCI-e (and for some very valid reasons).
However, new interfaces take time to get accepted, besides, if an old standard works perfectly fine, development of the product to switch to a new standard would be extra costs.
Unfortunately most computer motherboard manufacturers seem to want to follow the rush twords new technology even when older technology still works fine. What i don't understand is why do computer motherboard manufactures even put PCI-e x1 card slots in their systems. At the moment, the only technology that uses PCI-e slots is either graphics cards, raid controllers, or USB2.0. As far as graphics cards go, the PCI-e x16 slot is a necessary advancement. However, no one buys raid controllers because most people who use it have raid chips inside their motherboards anyways, this goes the same with USB2.0.
So whats the point of having PCI-e x1, x4, and x8 slots if they are rarely used? Instead of a useless PCI-e x1 slot, why not put in a useful PCI slot insted?
PCI-e x1 to PCI converters:
No, this is not possible. I have never seen a product for that purpose and I am not sure how they could do it. Pci-e slots(not even the full size 16x versions, let alone a 1x) are not backwards compatible with pci.
Edit: People are finding solutions for this. I do however agree they are not that cheap, add complexity (drivers, overhead, etc), and some of these adapters I am seeing will only work with low profile cards. I know some of these adapters will not work with all cases. You might have to remove the case mount plate on the PCI card to be able to use. If it is a full height PCI card, you might be stuck in many cases, unless you do not mind the side of your case being off.
They do and they work. I bought a simple slim one that adds about 1" height to the MB PCI slot (Tekit Desktop PCI-E1X TO PCI expansion card available at Newegg.com) and use it for my extremely old but necessary PCI modem that supports nextalk TTY software for the deaf. That older modem is the only one that will work with the software in windows 7 & up (not tested in Windows 10). Fortunately I have 3 of the older modems and taking care to keep them operating normally for over 15 years now. Now I can upgrade to newer motherboards that rarely have PCI ports (considering upgrading to newer GIGABYTE GA-X170-EXTREME ECC (rev. 1.0) LGA 1151 Intel C236) to that I can take advantage of NVMe M.2 ports, etc.
What should be happening is that MB makers should be including adapters that do just this--plug it in to convert it to PCI or PCIex1, etc. IN other words--backwards compatibility just like USB 3.0, 2.0, etc.
If in the future Windows 10 won't work with the software I can always run Windows 7 in a virtual windows to make use of the modem. Already tried that and it works.
Assuming that you're actually trying to plug a PCIe card to your desktop...
Double check your motherboard's manual. Sometimes you can plug a PCIe x1 board to a slot that is PCIe but more lanes than x1.
If you're trying to plug a PCI card on to a PCIe slot, that's quite different. You can probably still find a PCI to PCIe bridge somewhere, but it's not worth your while.
Is it possible to convert a PCI-e x1 slot into an old PCI slot?
No. PCI-e slots are completely different from PCI slot. Different pin-outs, different signals, different protocols. Converting PCI-e to PCI is like converting an electrical wall outlet to deliver water.
What you can do is use a PCI-e card with a PCI bus on it. This is an active card which speaks the PCI-e protocol and rather than adding popular things like network, graphics, RAID, SAS, etc you add a PCI bus. Then connect the your PCI card to this bus.
That is quite a bit different from 'convering' it.
Such adapters exist, for example the Startech PEX1PCI1 but there are some caveats.
The data interface is handled by a PCIe to PCI bridge chip. This part is not a problem, PCIe to PCI bridge chips are used on many motherboards and expansion cards and they work just fine.
Where it gets messy is mechanical and power considerations. The adapter card with the bridge chip on has a finite non-zero height. In the case of the Startech one it's designed so that using the adapter in conjunction with a low-profile card results in a regular height card that works fine if you have a low profile card with a low profile bracket. If you have a low profile card but with a full height bracket you may find it difficult to securely mount the card. If the actual card is full height then you may find it is not possible to access the connectors on the backplate or that it is not possible to fit the lid on your case.
Then there is power. PCI slots need 5V but PCIe slots don't provide it, so you end up with a cable hanging off the adapter board that needs to be connected to the power supply.
Finally the adapters are quite pricey, presumably because they are a niche product.
Often replacing the motherboard or card is likely to be a better option. The good news is that since this question was asked most types of expansion cards have become available in PCIe x1.